Liquid crystal displays (LCDs) have become an important attribute of portable devices because of their low energy consumption and small size compared to other contemporary devices. The development of the liquid crystal indicatory technology has enable LCDs to create color graphical images of high quality while maintaining their small size and weight, low energy consumption and relatively low price. These combined characteristics significantly broaden the applications of LCDs as displays and indicators of portable computers, computational systems and devices, as displays and indicators of measurement equipment and sensors, as displays and indicators of portable household devices such as mobile phones, onboard computers, notebooks, watches etc., as projectors and screens for large scale imaging in movie theaters, at shows, in public places and events, and as shutters in optical feedthroughs and sources of radiation.
The design of liquid crystal displays, principle of their operation and their main components have been described in literature. See, for example, Wu et al., “Reflective Liquid Crystal Displays” 2001, John Willey and Sons Ltd., and Lueder, “Liquid Crystal Displays: Addressing Schemes and Electro-Optical Effects”, 2001, John Willey and Sons Ltd.
In a mirror type or reflective type display, light enters the display and is reflected by a mirror and exits back through one side of the display. The main advantage of this type of display is the minimal requirements of artificial lighting in order to form an image. Generally, a mirror type display utilizes light from surrounding sources and does not require an illumination system.
This significantly decreases consumption of energy during operation. In order to operate the display in poor lighting or complete darkness conditions, the display system often includes an intrinsic source of illumination and optical components for front illumination of the display surface. But even in this case, the energy consumption of the illumination system is significantly lower than in the case of a transmitting display.
A combination of a reflecting and a transmitting type of displays is also common. This combined system is often designated a transflective type of display. The main distinction of the transflective type of display from the reflective type of display is that the mirror in the transflective type of display is semi-transparent and allows using the display in the transmitting regime if such regime is allowed by the design of the functional layers in the display.
Liquid crystal displays can be conveniently described in terms of rear and front sides. The front side is the one facing the viewer, while the rear side is the one opposite to the viewer. The set of layers in the display in front of the liquid crystal is referred to as the “front panel”, while the set of layers in the display behind the liquid crystal is referred to as the “rear panel”. The functional layers placed in the different panels are identified as the “rear” or “front” layers, for example, the rear and front substrates, and the rear and front electrodes, etc. The different sides of a single layer can also be identified in the display.
FIG. 1 schematically shows a reflective display comprising a set of flat functional layers performing various functions. In particular, the display comprises a front polarizer 101, a retardation plate 102, a front transparent substrate 103, a matrix of color filters 104, a front transparent electrode 105, a liquid crystal 106, a diffusive or specular or holographic reflector 107, and a rear transparent substrate 108. Numeral 109 represents the liquid crystal cell. In order to create an image on the display, light from the surrounding sources or illumination is modulated within the display's layered structure. In addition to the mirror and the source of light, particularly the functional layers of liquid crystal layer and at least one layer of polarizer form the image.
In a reflective display, as shown in FIG. 1, the liquid crystal is always situated behind the front polarizer, while the mirror is behind the liquid crystal. The principle of operation of the reflective liquid crystal display is based on controlling the state of polarization of light, polarized by the front polarizer, and changed by the nonlinear optical properties of the liquid crystal via application of voltage through the electrodes. The particular type of change of polarization of light at the exit from the liquid crystal depends on the operation regime of the liquid crystal in the display: twisted-nematic, super-twisted-nematic, or mixed mode. In case of twisted-nematic displays, the rotation of the polarization plane results primarily from the twist effect in the liquid crystal. In the case of super-twisted-nematic displays and displays with mixed-mode operation regimes, the change of the initial polarization state results from some combination of rotational twist effects and polarization phase retardation due to birefringence of the liquid crystal layer.
In reality, most modern liquid crystal displays rely on the mixed mode of operation, since the twisted- and super-twisted-nematic regimes require relatively large thickness of the liquid crystal layer, which may decrease image brightness. In transmitting displays, this decrease of brightness may be compensated by increasing the brightness of illumination source. However, in reflective displays such an approach does not work.
By changing the voltage across the liquid crystal, the state of polarization of light exiting the liquid crystal can be gradually changed. After the second interaction with the polarizer, the light intensity changes according to the value of applied voltage. The particular details of interaction of light with the liquid crystal in a reflective type display are determined by the selected regime of operation of the liquid crystal and its parameters. They also determine many performance characteristics of the display such as contrast ratio and brightness, viewing angle, transitional characteristic of the display, and achromatic color delivery, etc.
When the functional order of the main optical layers in a display is determined, the operation regime of the liquid crystal is determined by the mutual orientation of axes of each of the polarizers and the director of molecules of liquid crystal closest to the polarizer layer, the optical path difference between the ordinary and extraordinary rays in the liquid crystal, and the selected angle of twist of directors of molecules in the liquid crystal upon transition from one side of the crystal to the other. The presence of retarders and their characteristics also play a role. In almost all cases, the operation regime uses a certain combination of these parameters.
The common values of the angle of twist of liquid crystal are 45°, 90°, 240°, 270°. See Wu et al., “Reflective Liquid Crystal Displays”, 2001, John Willey and Sons Ltd. The angle of any polarizer is often chosen such that the transmission axis is parallel or perpendicular to the directors of molecules in the closest layer of the liquid crystal. When two polarizer layers are used, their transmission axes are often oriented to be perpendicular to one another.
Due to the fact that in the reflective display light goes through all the layers of the display twice, such displays may only have one polarizer. This polarizer is installed in front of the liquid crystal layer, thus the ray of light is polarized when it passes through polarizer for the first time. After the ray of light passes twice through the liquid crystal, before and after the reflection from the mirror, it interacts with the front polarizer again.
Reflective type displays with a single polarizer often have a poor contrast ratio. When the liquid crystal is operating in the mixed regime, light becomes elliptically polarized after it passes through the liquid crystal, which lowers the effectiveness of the second interaction with the polarizer.
Many publications address this problem. See for example, Kwok et al., “Generalized Parameter Space Diagrams For All Liquid Crystal Displays”, p.165-169, ASID 1999; Kwok, “Parameter Space Representation Of Liquid Crystal Display Operating Modes”, J. Appl. Phys., Vol. 80, No. 7, p.3687-93, October 1996; Cheng et al., “Dynamic Parameter Space Method To Represent The Operation Modes Of Liquid Crystal Displays”, Journal of Applied Physics, 86, p.5935, 1999. In order to increase the contrast ratio and enhance other characteristics of the display, the prior art references suggest varying value of all parameters which determine the operation regime of the liquid crystal, including optical path differences in the liquid crystal, angles of turn of the polarizer relative to the directors of molecules in the peripheral layer of the liquid crystal, as well as the angle of turn of the directors of molecules in the liquid crystal. EP0576303 and U.S. Pat. No. 6,108,064 suggest the use of retarders as functional layers for the same purpose.
As a result of calculations and experiments in this direction, acceptable values of contrast ratio with relatively large viewing angles have been derived for color displays as well as for black and white displays, see for example, EP985953, U.S. Pat. Nos. 5,926,245 and 6,341,001. However, using untraditional parameters of operation regime of the liquid crystal often leads to complications in their design, which often makes the mentioned results poorly reproducible in mass production of displays.
This difficulty can be illustrated in a particular example related to one of the most often varied parameter: the angle between the optical axis of the polarizer and the directors of molecules of the liquid crystal in the layer closest to the polarizer. The direction of the transmission axis of the polarizer based on dichroic organic molecules is fixed at the time of polarizer alignment. In most cases, this fabrication step comprises stretching a ribbon of polarizer material through a special device. As a result, the direction of the axis of the polarizer comes out parallel to the edges of the ribbon. If the mentioned angle is not either 90° or 0°, then when the ribbon is cut to size before being installed into the display, the amount of wasted material is increased.
U.S. Pat. No. 6,417,899 describes a known color liquid crystal display comprising an internal polarizer placed between an alignment layer and a layer of color light filters. The drawback of such a display is that it uses the alignment layer to define the direction of the transmission axis of the polarizer, which complicates fabrication of the display, and excludes using operation regimes when the axis of the polarizer is not parallel to the axis of the alignment layer. Further, there is a possibility of worsening the image quality in case of poor alignment of the light filter and liquid crystal layers.
Uchida discloses a reflective type color liquid crystal display without the internal polarizer wherein the distance between the layer of color filters and mirror is reduced at the expense of eliminating the rear polarizer in order to achieve the proper color delivery when the display is observed at an angle. See Uchida, Reflective LCDs, SID Seminar Lecture Notes, Hynes Convention Center, Boston, 20-24 May 2002, Vol. II, p F2/3. One of the drawbacks of such a design is the decreased contrast ratio, which is unavoidable for reflection type displays without rear polarizer. In addition, the matrix of color filters in such a display is placed in front of the liquid crystal, which decreases the viewing angle due to the increased viewing parallax resulting from the increase of the distance between the matrix and the mirror.